1. Field of the Invention
The present invention relates generally to a technique for precisely deriving wheel speed data which is used in an anti-skid brake control, slip control and so forth. More specifically, the invention relates to system for correcting wheel speed data which is derived on the basis of the output of a wheel speed sensor, for avoiding the influence of drop output of sensor pulse, noise superimposed on the sensor pulse and so forth.
2. Description of the Background Art
Wheel speed data is important parameter for providing accuracy in anti-skid brake control, slip control. For obtaining accurate wheel speed data, various systems have been developed and proposed.
For example, U.S. Pat. Nos. 4,674,049, 4,704,884, 4,663,715, 4,663,716, 4,660,146, 4,665,491, 4,780,818, 4,674,050, 4,680,714, 4,682,295, 4,680,713, 4,669,046, 4,669,045, 4,679,146, 4,656,588, 4,718,013, 4,569,560, 4,662,686, 4,667,176, 4,597,052, 4,730,156, 4,637,663 and 4,683,537 respectively issued on June 16, 1987, Nov. 3, 1987, May 5, 1987, May 5, 1987, Apr. 21, 1987, May 12, 1987, Oct. 25, 1988, July 14, 1987, July 21, 1987, July 14, 1987, June 16, 1987, May 26, 1987, July 7, 1987, Apr. 7, 1987, Jan. 5, 1988, Feb. 11, 1986, May 5, 1987, May 19, 1987, June 24, 1986, Mar. 8, 1988, Jan. 20, 1987 and July 28, 1987, all have been assigned to the common assignee to the present invention. In the above-identified U.S. patents, discloses anti-skid brake control system which incorporate technologies of monitoring a wheel speed by means of a wheel speed sensor. The wheel speed sensor comprises a rotor rotatable with a vehicular wheel to monitor the vehicle speed, and a sensor element, such as magnetic proximity switch, optical sensor or so forth.
The disclosed wheel speed sensor generates wheel speed indicative signal of alternating current form. The alternating frequency of the wheel speed indicative signal is variable depending upon the wheel speed. Wheel speed indicative pulse trains an generated by shaping the alternating current signal. Based on the pulse signal, wheel speed data, wheel acceleration data and so forth are derived. However, in such construction, it is possible to cause drop out of the alternating current signal due to distruction of the magnetic field in case of the magnetic proximity switch. On the other hand, it is also possible to superimpose noise on the wheel speed indicative pulse train. In such case, error in the wheel speed indicative data will cause error in anti-skid brake control or slip control.
In order to obtain higher precision in anti-skid control or slip control, it is essential to obtain precise wheel speed data which should be free from distruction of the magnetic field in the magnetic sensor or noise. However, as long as the magnetic sensor is used in monitoring the wheel speed, it is inevitable to subject destruction of the magnetic field. Also, it is not possible to avoid noise superimposed on the sensor signal.